Severing an electrically conductive link of an integrated circuit (IC) chip and trimming thin film on a silicon wafer are two examples of laser processing of light reflective multilayer structures. One of the major challenges associated with laser processing of such process targets is that the constructive or destructive interference effect stemming from different thicknesses of the multiple passivation and target material layers affects the percentage of laser energy that can be effectively absorbed by the link or thin film process target. This in turn affects the process result when constant laser pulse energy is used to process the process target. In some cases, when the interference effect reduces the percentage of laser energy absorbed by the process target, incomplete blowing of the link or trimming of the thin film results in a failed process. In some other cases, when the interference effect increases the percentage of laser energy absorbed by the process target, over-processing results in poor process quality such as over-blowing a passivation layer. Interference effects significantly affect the risk of damage to the silicon wafer as well. Variation of the interference effect is related to the variation of passivation layer thickness for a given number of passivation layers, passivation layer material, link or thin film thickness and material, laser wavelength used, and other such factors. Unfortunately, it is very difficult to make a passivation layer thickness uniform across the surface of a wafer or among multiple wafers in a group of wafers. This makes laser processing inconsistent and difficult to control. Moreover, the overlying passivation layer thickness and material type affect the link blowing process because the rupture behavior depends on these characteristics.
Interference effects on laser processing resulting from multiple layer structure parameter variation, including passivation layer thickness, have been reported and analyzed in detail by a variety of publications, including applicant's PhD thesis, titled: “Laser processing optimization of semiconductor based devices,” YL Sun, Oregon Graduate Institute of Science & Technology, 1987 (Sun thesis). The Sun thesis also analyzed in detail the impact on memory chip link processing of different thicknesses of the overlying passivation layer.
U.S. Patent Application Pub. No. 2006/0216927 A1 of Cordingley, et al. ('927 publication) proposed adjusting laser pulse energy by measuring at least one of a thickness and reflectance of a layer to determine a variation of a laser pulse characteristic. The '927 publication suggested several approaches to determining such variation, including (1) simply measuring reflectance from the target, (2) measuring the passivation layer thickness and material refractive index with ellipsometry or spectral analysis, and (3) measuring reflectance at two different wavelengths and calculating the thickness of the oxide (i.e., the passivation layer). There are problems associated with each of these proposed approaches.
For proposed approach (1), in the case of link processing, the link width is about 1.0 μm. The smallest practicable laser beam spot size for a laser processing system is typically larger than the link width, therefore, a simple reflectance measurement with the laser beam or other system light source provides a mixture of reflectance contributions from the link structure and from the neighboring structure on the silicon wafer with no link present (impacted by all of the different passivation layers stacked over them). The '927 publication stated that positioning the laser spot relative to the link twice in different locations enabled derivation of reflection information from the link portion. The teaching of the '927 publication was to use only this reflection information from the link to control the processing laser parameters. As discussed in the Sun thesis, however, the reflection information from the multilayer structure neighboring but not including the link and the passivation layer structure underlying the link also plays very significant roles in the successful link processing. Contrary to what was suggested by the '927 publication, a higher reflectance measured from the top surface of the link does not necessarily dictate higher laser energy to achieve better process quality.
For proposed approach (2), the problem is that the measurement is performed off line with ellipsometer or spectral analysis equipment, which off-line measurement increases manufacturing cost and is very time consuming. Moreover, these techniques can work when there is only one layer overlying the link and when the complex refractive index of the link material is well known. For acceptable accuracy, the measuring beam spot size has to be smaller than the link width. This is impractical with most commercially available ellipsometers and spectral analysis equipment. For the case in which there are multiple passivation layers overlying the link, or in which the complex refractive index of the link material is unknown, it is almost impossible to determine the thickness of each layer and its refractive index. On the other hand, if successful, this measurement provides information about only the passivation layer overlying the link. This measurement does not provide information about risk of laser damage to the silicon wafer in the area neighboring the link because the issue of passivation layers stacked over the wafer surface region unoccupied by the link is not addressed.
For proposed approach (3), there are two different lasers or light sources required for making the measurement. The '927 publication stated that, when the reflectance at two different wavelengths is measured, the passivation layer thickness can be calculated with the known refractive index of the passivation layer material. Again, this proposal fails to realize that, to make the proposed practice feasible, one has to meet an impracticably realizable condition that the light spot size be smaller than the link width. There is also required knowledge of the complex refractive index of the link material. The measured reflectance of the overlying passivation layer represents not only the behavior of the overlying passivation layer itself, but also the behavior of its interface with the link, as taught by the Sun thesis. Moreover, whenever the link material, such as polysilicon, is partly transmissive of laser output, the underlying passivation layer also contributes to the measured reflectance.
The '927 publication acknowledged the issue of link width and laser beam spot size and proposed performing more measurements with the laser beam spot shifted to different locations relative to the link to derive reflection information by the link structure. This approach is tedious, requiring a long time to carry out. More importantly, the '927 publication failed to teach or appreciate the importance of the layer structure neighboring the link on the link laser processing and laser parameter selection or the importance of the underlying passivation layer structure on the link processing. Therefore, selection of laser pulse characteristics based on the thickness and reflectance measurements of a layer over a target structure, as taught by the '927 publication, would afford no warranted improvement of the laser processing.